Step 2: Change Runtimes

Step 3: The ART RUNTIME

Android Runtime (ART) is an applicationruntime environment used by theAndroid mobile operating system. ART replaces Dalvik, which is the process virtual machine originally used by Android, and performs transformation of the application'sbytecode into native instructions that are later executed by the device's runtime environment.[1]

Unlike Dalvik, which since Android 2.2 "Froyo"uses just-in-time (JIT) compilation to compile the bytecode every time an application is launched,[2] ART introduces use of the ahead-of-time (AOT) compilation by performing it upon the installation of an application. By reducing the overall amount of compilation that needs to be performed across the operation of an application, a mobile device's processor usage is reduced and battery runtime is improved. At the same time, ART brings improvements in performance,garbage collection, applications debuggingand profiling.[1][3]

To maintain backward compatibility, ART uses the same input bytecode as Dalvik, supplied through standard .dex files as part of APK files, while the .odex files are replaced with Executable and Linkable Format (ELF) executables. Once an application is compiled by using ART's on-device dex2oat utility, it is run solely from the compiled ELF executable; this approach eliminates various overheads involved with JIT compilation, but it requires additional time for compilation when an application is installed, and applications take up slightly larger amounts of space to store the compiled code.[1][3]

A technology preview of ART debuted as an alternative runtime environment in Android 4.4 "KitKat".[4][5] In the next major Android release, Android 5.0 "Lollipop", Dalvik was entirely replaced by ART.[6]